water Article Application of Open Source Electronics for Measurements of Surface Water Properties in an Estuary: A Case Study of River Jadro, Croatia Vladimir Divi´c 1 , Morena Galeši´c 1,* , Mariaines Di Dato 2 , Marina Tavra 1 and Roko Andriˇcevi´c 1,3 1 Faculty of Civil Engineering, Architecture and Geodesy, University of Split, 21000 Split, Croatia; [email protected] (V.D.); [email protected] (M.T.); [email protected] (R.A.) 2 Helmholtz Centre for Environmental Research—UFZ, Department of Computational Hydrosystems, 04318 Leipzig, Germany; [email protected] 3 Center of Excellence for Science and Technology-Integration of Mediterranean Region, University of Split, 21000 Split, Croatia * Correspondence: [email protected]; Tel.: +385-95-564-0784 Received: 28 October 2019 ; Accepted: 7 January 2020 ; Published: 11 January 2020 Abstract: There are multiple factors affecting the behavior of water properties in an estuary, including the hydraulic properties of rivers and corresponding receiving water bodies, along with the potential solutes brought by rivers. Although there are various numerical models and analytical approaches to solving particular or holistic problems in estuaries, measurements are inevitably required. In this study, we developed an innovative low-cost probe based on the Arduino platform as an alternative to more expensive measuring systems. Our device is designed to measure position, temperature, and electrical conductivity in multiple realizations, and it consists of a floating container equipped with the following components: an Arduino Mega development board, a power management module, an SD card logging module, a Bluetooth module, a temperature measuring module, a global positioning satellite (GPS) position module, and a newly developed module for measuring electrical conductivity (EC). We emphasize that all used tools are open-source and greatly supported by the worldwide community. We tested our probe during a field campaign conducted at the estuary of River Jadro near Split (Croatia). Nine probes were released at the river mouth and their position, temperature, and EC were monitored and recorded during the experiment, which ended when the probes stopped, due to the river plume attenuation. The same experiment was repeated three times. All of the probes recorded consistent temperature data, while the EC data show more variable behavior, due to the higher sensitivity of the corresponding sensor. This was expected as a part of the natural process in the estuary. The measured data were additionally used to parameterize an analytical model for mean flow velocity and salinity as a proxy concentration. This showed a good match between the experimental results and the theoretical framework. This work, although focused on water surface applications in the near field zone of an estuary, should be considered as a promising step toward the development of innovative and affordable measurement devices. Keywords: estuaries; field measurements; Arduino; low-cost measurement system; salinity importance; conservative solute proxy 1. Introduction Estuaries are dynamic ecosystems that experience constant environmental changes due to their nature of being a transition zone among land, river, and sea. Historically, estuarine science has even been considered as a “poor relation” between freshwater and marine science [1]. However, among all Water 2020, 12, 209; doi:10.3390/w12010209 www.mdpi.com/journal/water Water 2020, 12, 209 2 of 29 the surface water resources, estuaries and coastal ecosystem represent one of the richest and the most threatened natural systems globally [2,3]. The importance of estuaries is reflected in the value they deliver through their ecosystems, ranging from direct consumable benefits to indirect benefits that support the local biome and geophysical processes [4–7]. In an attempt to protect and preserve estuarine values, environmental directives and guidelines have incorporated estuaries within multiple regulations. For instance, in the United States, estuaries are covered by the National Estuary Program within the Clean Water Act [8], while, in the European Union (EU), they are well represented in the Water Framework Directive (WFD), the European Bathing Water Directive (EUBWD), and the Marine Strategy Framework Directive (MSFD) [9–11], and are specifically treated as transitional waters. However, most of these programs have the same obstacle when it comes to implementation, and that is the scarcity of data, in particular, spatial data, which has already been covered by multiple studies [12–15]. The INSPIRE (Infrastructure for Spatial Information in Europe) Directive [16] addresses this issue by acting as an umbrella platform covering the spatial data infrastructure for EU environmental policies and activities. In particular, Marine Spatial Data Infrastructure (MSDI) is focused on coastal areas [17] where it is closely related to both WFD and MSFD. Similar to other Member States, Croatia has introduced MSDI into the national legislative framework within the fully functional National Spatial Data Infrastructure (NSDI). Furthermore, a planning support concept was applied [18] and key stakeholders have identified priority data themes, but there are few data available online at this moment [19]. Consequently, MSDI development should follow monitoring efforts to enable better availability of spatial data and inter-operability. The monitoring requirements issued by the above-mentioned regulations are pressing Member States and corresponding environmental agencies to assign a significant amount of resources to obtain data on water properties. The goal is to continuously assess the ecological status of certain water bodies and work on an improvement if needed [20]. When analyzing the concentration distributions and dilution processes, which are intrinsically random, corresponding field experiments of repetitious realizations for dye release are quite expensive and tedious. In particular, Riddle and Lewis [21] studied the mixing process in coastal waters by collecting and analyzing tracer dye experiments in 37 locations (among them, 25 were from the UK, five were from beaches in Malaysia and Singapore, and the last two were from France and China). Their datasets were composed of a total of 285 dye experiments recorded between 1968 and 1996. In addition, Rodriguez et al. [22] carried out a field experiment at the Delta Ebro surf zone, in Spain, over 13–17 December 1993. Their experimental design comprised simultaneous quantification of several parameters, such as incident waves, spatial velocity, and dispersion of five types of dyes. The process was repeated for eight cases. Later, Clarke et al. [23] quantified the dispersion coefficients in Santa Monica Bay by analyzing the statistics of 14 dye releases from 1999 to 2000. Continuous research has also been done to predict some of the parameters without interfering in the environment itself, for instance in micro-tidal, river dominated estuaries in Croatia [24,25] with the focus on the distribution of potential conservative pollution brought to coastal waters by rivers, or in a broader sense in New Zealand estuaries [26]. Among different water properties in estuaries, salinity is one of the most studied [27,28], both for its importance to the ecosystem (hydrodynamics and biome), and for its cost-effective measurement as opposed to, for instance, biological parameters. For instance, Wiseman et al. [29] analyzed the salinity trend of two historical datasets from estuarine waters in south Louisiana (USA); their study shows significant trends in mean salinity, salinity variance, and maximum salinity. Later, Bradley et al. [30] modeled the salinity decrease caused by a discharge reduction at Cooper River (South Carolina, USA), which occurred in 1985, thereby observing considerable changes in estuarine plant distribution. In a recent paper, Lorenz [31] reviewed the data on vertebrate fauna at Florida Bay (USA), finding a strong correlation between decrease in estuarine population and increase in salinity level. Similar relationships between salinity concentration and estuarine ecosystems indicators were observed by Spalding and Hester [32] for coastal Louisiana (USA); by Rivera-Monroy et al. [33] at Ciénaga Water 2020, 12, 209 3 of 29 Grande de Santa Marta–Pajarales Lagoon Complex (Colombia); and by Little et al. [34] in Southern England (UK). Moreover, salinity may be used to determine multiple hydrological properties indirectly (e.g., coefficient of diffusion, velocity, and concentration statistics), as shown by several authors, such as Vallino and C.S. Hopkinson [35] for Plum Island Sound estuary (Massachusetts, USA); Ho et al. [36] for the estuary of Hudson River in New York (USA); Gay and O’Donnell [37] for the Mid-Atlantic (USA); Xu et al. [38] for Chesapeake Bay (USA); Ref. [39] in the coastal zone of north-eastern Japan; and Andriˇcevi´cand Galeši´c [40] for the River Žrnovnica in Split (Croatia). This approach has been analyzed [40,41], by measuring the salinity at different points in time with a commercial probe, the Sea-Bird’s SBE 37-SI MicroCAT. Depending on the final goal of the comparison between measured and modeled data, different spatial configurations of the measured points were previously planned and executed accordingly within four measurement campaigns on the River Žrnovnica [42]. We emphasize that measured salinity data were well aligned with the theoretical framework based on the analytical model for solute concentration statistics. Results